Conversion and containment of radioactive organic liquids into solid form



United States Patent 3,463,738 CONVERSION AND CONTAINMENT OF RADIO- ACTIVE ORGANIC LIQUIDS INTO SOLID FORM Curtis L. Fitzgerald, Kingston, Herschel W. Godbee, Oak Ridge, and Kenneth H. McCorkle, Jr., Powell, Tenn., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed May 1, 1968, Ser. No. 725,890

Int. Cl. G21f 9/20 US. Cl. 252-3011 4 Claims ABSTRACT OF THE DISCLOSURE Radioactive organic liquids are effectively converted into solid form by homogenously mixing liquefied polyethylene with the radioactive liquids and cooling the resulting mixture to form a solid essentially non-porous, rigid polyethylene body which effectively contains the radioactivity.

BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the US. Atomic Energy Commission.

The processing of nuclear fuels by solvent extraction techniques to separate uranium, thorium, plutonium, and other useful products results in the generation of relatively large volumes of radioactive organic liquids. These liquids generally comprise an organic extraction reagent selected from alkyl phosphates, alkyl phosphoric acids, long chain amines, or quaternary ammonium compounds dissolved in an organic solvent selected from saturated paraffin and aromatic hydrocarbons. Corrosive materials such as nitric, phosphoric, or sulfuric acids are commonly found in these organic wastes in dissolved or dispersed form. Present methods for disposal or containment of these radioactive organic volumes consist of storage in large tanks, burning, or seepage into suitable geologic formations. Each of these methods suffers from major disadvantages. Storage in tanks requires the use of expensive corrosion resistant materials and constant vigilance to monitor any seepage of radioactivity. Burning presents a major air pollution problem, and seepage into geologic formations presents the ever constant danger of polluting water supplies. None of these methods are entirely acceptable from a health and safety point of view. As reactors are designed for higher fuel burn-up, large volumes of radioactive liquids containing higher quantities of radioactivity will inevitably be generated in the future, thus, magnifying the problem.

SUMMARY OF THE INVENTION With the above statement of the problem in mind, it is an object of this invention to provide an effective method for converting radioactive liquid volumes of the nature described into a solid, essentially non-leachable form. It is another object of this invention to convert radioactive liquid volumes into a form which eradicates or reduces the potential for pollution of the atmosphere or of geologic formations. Other objects within the same scope of this invention will be apparent from the ensuing description.

The present invention is based on the discovery that liquid polyethylene compositions can form homogeneous mixtures with radioactive volumes of the character described which, when solidified, form a solid, rigid, essentially non-porous mass which efiectively contains the radioactivity. The resulting polyethylene mass is physically and chemically stable over a wide range of conditions and can be stored either above or below the ground. In either case, the pollution potential will have been considerably reduced in comparison to the prior art methods for dealing with the same problem.

The method of this invention requires the formation of a liquefied flowable polyethylene product. For the purpose of this invention, a low molecular weight polyethylene having a melting point in the range 50 C. to no more than 200 C. is required. The material can be used in any liquid form, such as molten plastic, as an organic sol, as a solution in an organic solvent, or as an aqueous emulsion of polyethylene. In terms of providing an effective radioactive containment medium, however, the various forms of liquefied polyethylene are essentially equivalent. In order to practice this invention any one of the several forms of liquefied polyethylene is homogeneously mixed with the radioactive organic feed solution until a homogeneous mixture is obtained.

This invention may be practiced in a manner which takes full advantage of the organic solvent retention capacity of polyethylene to incorporate virtually the entire organic feed into the resultant solid polyethylene mass. The exact volume or weight capacity of the polyethylene will vary according to the molecular weight of the polyethylene and the composition of the radoactive organic feed. In general terms, however, we find that the liquefied plastic can incorporate up to 30 percent by weight of feed organic material into a solid non-bleedable polyethylene mass. On the other hand, it may be found desirable in cer tain instances to remove portions of the organic liquid by volatilization in cases where, for example, it is found desirable for economical and other reasons to recover the organic solvent. In such cases, homogeneous mixing of the feed organic solution with the liquefied polyethylene is conducted under such conditions as to effect vaporization of the organic solvent. In general terms, however, maximum radioactivity retention in the final polyethylene mass will occur under conditions which retain a minimal amount of organic solvent. However, this does not place any real limitation upon the process since if both maximum organic solvent and radioactivity retention are desired, it is merely necessary to increase the volume of the initial polyethylene feed to accommodate bothmaximum radioactivity and solvent retention.

The apparatus used to form the desired homogenous mixture of polyethylene and organic feed are those which are conventionally available in the chemical engineering art and may consist of a mechanically stirred evaporator of the thin film type designed to operate at a temperature of more than about 200 C. A thin film evaporator is preferred so that rapid and eflicient evaporation can take place over a short period of time at a minimal temperature. Evaporation temperatures of no more than 200 C. are advisable in order to minimize any possibility of polymer degradation.

The following examples are designed to illustrate a typical embodiment by which this invention may be practiced and includes experimental data which establishes the utility of the final product as an effective repository for radioactive values contained in organic solutions.

EXAMPLE I One hundred grams of a low molecular weight polyethylene in granular form was heated to C. under a reduced pressure of about 30 ml. absolute. To this liquid material was fed about ml. of a simulated organic waste at a rate of 3 ml./min. consisting of 70 volume percent dodecane and 30 volume percent tributylphosphate. After mixing to form a substantially homogeneous texture, the temperature of the evaporator was increased to 130 C. and evaporation was continued until the dodecane was volatilized. The pressure in the evaporator was then reduced to atmospheric and the liquid residue was drained out, cooled to room temperature whereupon a solid, rigid body hardly distinguishable from any other mass of solid polyethylene was formed.

EXAMPLE II An organic feed solution containing 30 weight percent tributylphosphate and 70 weight percent polyethylene spiked in one case With ruthenium-106 and in another with zirconium-95 and a radioactive isotope of niobium was treated in accordance with the procedure described in Example I except that no attempt was made to etfect vaporization of the organic solvent. The resulting solid homogeneous polyethylene masses issuing from the evaporator unit was then leached with distilled Water for 36 days. At the end of 20 days the fraction of original Zirconium-niobium leached from the polyethylene mass was l.49 l- (cm. /g.) day-l where cm. refers to the surface area of the leached specimen, and g. is the weight in grams of the polyethylene mass. Thus, assuming a specific surface area of 2 square centimeters this means that 3X10 fraction of the original activity was leached per day. At the end of 36 days the fraction of original ruthenium-106 leached from a unit of specific surface area of polyethylene per day was 434x" (cm. /g.)- day- Since the major portion of radioactivity normally found in organic efiluent streams issuing from nuclear fuel solvent extraction processes are represented by ruthenium-106 and zirconium-niobium, it is seen that the leaching tests are representative of the result obtainable by using the method of this invention. The magnitude of leach rate from the resulting polyethylene composition is so sufliciently low as to allow storage of these polyethylene masses either above or below the ground. In cases where storage below the ground is desired, it would be preferable to store them above the water table of the particular geologic area.

It will, therefore, be noted that we have described and demonstrated a process for converting large volumes of radioactive organic liquids into solid polyethylene masses which effectively contain not only the radioactivity but can be utilized to retain the entire liquid volume of the organic feed. The method takes advantage of the physical and chemical stability of polyethylene and takes particular advantage of the radiostability of polyethylene compositions which can experience a total dosage of up of 10 rads. without degradation or gas formation. The method of this invention is particularly noteworthy as providing a means for containing radioactivity from organic waste since it does not suifer from any of the disadvantages characteristic of the prior art methods for dealing with the same problem. It should be noted, for example, that the final polyethylene mass requires little or no monitoring once it is stored in a suitable location in contradistinction to the continual care required when such volumes are storedv in tanks. Little or no possibility exists for air pollution, and release of radioactivity to water streams whenever stored in underground geologic areas either above orbelow the water table is remote. The leaching data confirmed that even Where Water leaching would be possible, the rate of radioactive release is within the prescribed health and safety standards.

While we have described this invention in terms of utilizing polyethylene alone as the radioactive retention medium, it is within the scope of the disclosed inventive concept to incorporate other thermoplastic polymers in combination with polyethylene wherever physical modification such as viscosity, solvent retaining capacity, melting point range, and other parameters require modification beyond that available by the use of polyethylene alone. Among those modifying polymers which may be used are methyl methacrylate, polypropylene, and polystyrene.

What is claimed is:

1. A method of converting a radioactive organic solution into a solid form which comprises forming a homogeneous mixture of said solution with liquid polyethylene at a polyethylene/ solution Weight ratio suflicient to at least contain the radioactivity of said solution, and then cooling the resultant liquid mixture to a solid.

2. The method according to claim 1 wherein the polyethylene is liquefiable at a temperature in the range C. to 200 C.

3. The method according to claim 2 in which the polyethylene is in the form of a sol.

4. The method according to claim 1 in which the ratio of polyethylene/feed organic liquid is sufficient to retain the radioactivity as well as the organic solution when the liquefied polyethylene is cooled to solid form.

References Cited UNITED STATES PATENTS 3,366,716 1/1968 Cohen 25230l.l X

CARL D. QUARFORTH, Primary Examiner MELVIN J. SCOLNICK, Assistant Examiner 

